1. Field of the Invention
The present invention relates to a commercially applicable composite reverse osmosis membrane which has improved water-permeability, salt rejection rate, chlorine resistance, and so forth. More particularly, the invention relates to a composite reverse osmosis membrane which comprises a porous support layer and a superposed active layer of a cross-linked aromatic polyester or copolymer of aromatic polyester and aromatic polyamide.
2. Description of the Prior Art
Processes for removing salts from seawater, brine, waste water, and so forth by means of a reverse osmosis separation membrane have gained wide attention in view of the economics of energy and resources, environmental protection and so on.
The first composite reverse osmosis membrane, an asymmetrical membrane formed of cellulose acetate, is disclosed in U.S. Pat. Nos. 3,133,132 and 3,133,137 issued to Leob and Sourirajan in 1962. However, this separation membrane was susceptible to hydrolysis and not sufficiently resistant to microbial attack and chemical substances. In addition, in order to increase the water-permeability, it was necessary to lower the resistance to the operating pressure and durability to a significant degree, and thus, such membrane has been applicable only in limited fields.
Many attempts have been made to develop reverse osmosis membranes of new materials which do not suffer from the defects of those membranes prepared from cellulose acetate: aromatic polyamide or polyamide hydrazide membranes as disclosed in U.S. Pat. No. 3,567,632; polyamide acid membranes as disclosed in Japanese Patent Publication No. (sho) 55-37282 and U.S. Pat. No. 3,878,109; crosslinked polyamide acid membranes as disclosed in Japanese Patent Publication No. (sho) 56-3769; polysulfone amide and polyarylene oxide membranes. However, those membranes did not provide any remarkable advantages over the cellulose acetate membrane in terms of their performance.
Unlike the above asymmetric membranes, composite membranes having a porous support layer and an ultrathin active layer superposed thereon, which plays a critical role in the separation performance of membranes, have been developed and widely used in the field of the separation membranes. These membranes can be optimized by properly selecting the materials for the active and porous support layers, and also can be advantageously stored and transported in a dry state. This differentiates such membranes from the prior art asymmetric membranes, which have to be stored and transported in a wet state.
There are two types of composite membranes: one is a composite membrane comprising a support layer and an active layer superposed thereon, as taught and exemplified in U.S. Pat. Nos. 3,744,642, 3,926,798, 4,277,344, 4,557,949, and 4,366,062, and Japanese Laid-Open Patent Publication Nos. (sho) 55-147106 and (sho) 58-24303, etc.; and the other is a composite membrane comprising a support layer, an active layer, and a gel layer interposed therebetween for the purpose of reinforcing the mechanical strength of the membrane, as suggested in U.S. Pat. Nos. 4,039,440, 3,951,815, 4,005,012 and 4,559,139, and Japanese Laid-Open Patent Publication No. (sho) 49-133282, and Japanese Patent Publication Nos. (sho) 55-49,524 and 55-38,164.
However, these conventional composite membranes do not fulfill the requirements for reverse osmosis separation such as water-permeability, salt rejection rate, chlorine-resistance, heat-resistance, chemicals-resistance, pressure-resistance, oxidation stability, etc. For example, U.S. Pat. No. 3,904,519 discloses a process for the preparation of an active layer by the interfacial polymerization of an acyl halide having two reactive groups and an aromatic amine having two reactive groups, or by the reaction of the two amine reactive groups and an aromatic amine having a carboxylic group. However, this product does not provide satisfactory results in terms of water-permeability, chlorine-resistance, oxidation stability, etc. To increase chlorine-resistance and oxidation stability, a composite film type reverse osmosis separation membrane has been prepared from an aromatic amine having two or more reactive groups and an acyl halide or cyclohexane tricarbonyl chloride having two or more reactive groups (See, U.S. Pat. Nos. 4,277,344, 4,606,943, 4,643,829 and 4,626,468; and European Patent Application No. 86 3059440). Those membranes also do not provide satisfactory results in terms of membrane properties including water-permeability, chlorine-resistance, and oxidation stability.